Recent years have seen progress in the development and practical implementation of display devices (hereafter referred to as organic EL display devices) using organic EL elements. Generally, an organic EL display device includes (i) a display unit having, arranged in a matrix, plural pixel circuits each having an organic EL element, and (ii) a drive circuit for driving the display unit.
The active-matrix organic EL display devices which have been put into practical implementation have a structure (cathode common structure) in which cathode electrodes of organic EL elements of respective pixels have a common potential. Furthermore, drive circuits for controlling the light emission of these organic EL elements are generally configured of P-type thin film transistors (TFT).
On the other hand, a high performance TFT has been explored in an attempt to enhance performance of an amorphous silicon TFT which is easy to manufacture. For example, an oxide TFT which uses the oxide as the semiconductor layer is almost ready for practical use and is gathering attention. The characteristics of the oxide TFT have been realized only for the N-type TFT, which is reported to have mobility of 10 times or more than the amorphous silicon transistor.
In such a situation, what is important in providing high-performance display devices while saving the manufacturing costs is a pixel circuit of N-type transistor that supports the cathode common organic EL element which is technically established and causes the organic EL element to emit light at a more accurate and stable luminance. Thus, various circuit structures and control methods have been proposed (see Patent Literature (PTL) 1, for example).
FIG. 20 is a circuit diagram showing a conventional pixel circuit 90 disclosed in PTL 1. The pixel circuit 90 includes a drive transistor TD, a switching transistor T9, a capacitor Cs, and an organic EL element EL. The pixel circuit 90 includes two transistors and one capacitor only, and is able to cause the organic EL element to emit light at an accurate and stable luminance.
The pixel circuit 90 is supplied with a control signal from a scanning line drive circuit 4 via a signal line SCAN, and supplied with a data voltage corresponding to a light-emitting luminance from a signal line drive circuit 5 via a data line DATA. Furthermore, a power source voltage for use in the light emission by the organic EL element EL is supplied to the pixel circuit 90 from a power source circuit not shown in the Drawing via power source lines VDD and VSS.
FIG. 21 is a timing chart showing an example of the control signal, data voltage, and power source voltage for operating the pixel circuit 90, for one-frame period. In FIG. 21, the vertical axis denotes the level of each signal, and the horizontal axis represents the passing of time. To facilitate description, the control signal, the data voltage, and the power source voltage are given the same names as the respective signal lines and power source lines through which they are transmitted.
The pixel circuit 90 repeats a Vth detecting step, a data writing step, a resetting step, and a light-emitting step, on a frame basis, according to the control signal, power source voltage, and data signal shown in FIG. 21.
In FIG. 22, (a) to (d) are circuit diagrams for describing the operation on the pixel circuit 90 in the Vth detecting step, the data writing step, the resetting step, and the light-emitting step, respectively.
The operation on the pixel circuit 90 performed according to the control signal, data voltage, and power source voltage shown in FIG. 21 is described with reference to (a) to (d) in FIG. 22.
As shown in (a) in FIG. 22, in the Vth detecting step, the power source voltage VDD is set to 0, the power source voltage VSS is set to VE2, and the data voltage DATA is set to VDH. There is a conducting state between the switching transistor T9 and the drive transistor TD, and the voltage of the gate electrode of the drive transistor TD is converged to the voltage Vth which is a voltage increased from the power source voltage VDD by the threshold voltage Vth of the drive transistor TD. The threshold voltage Vth is held by the capacitor Cs based on the voltage VDH obtained from the data line DATA.
As shown in (b) in FIG. 22, in the data writing step, the data voltage DATA is set to a voltage dropped from VDH by ΔVdata that is an amount corresponding to the light-emitting luminance. Since the switching transistor T9 is in the conducting state, the drop amount ΔVdata of the data voltage DATA is distributed to the capacitor Cs and the parasitic capacitance Cel of the organic EL element according to the ratio between the capacitance of the capacitor Cs and the parasitic capacitance Cel of the organic EL element EL: Cel/(Cel+Cs)=α. As a result, the voltage of the gate electrode of the drive transistor TD becomes −αΔVdata+Vth.
As shown in (c) in FIG. 22, in the resetting step, the switching transistor T9 is switched to the non-conducting state (represented with dashed line). Furthermore, the power source voltage VDD is set to −VE1, and the data voltage DATA is set to VDH again. Since the switching transistor T9 is in the non-conducting state, the increase amount ΔVdata of the data voltage DATA is all added to the voltage of the gate electrode of the drive transistor TD. As a result, the voltage of the gate electrode of the drive transistor TD becomes (−αΔVdata+Vth)+ΔVdata=(1−α)ΔVdata+Vth.
The drive transistor TD is turned into ON state by the differential voltage between the voltage of the gate electrode and the power source voltage VDD, and the anode voltage of the organic EL element EL is initialized to −VE1.
As shown in (d) in FIG. 22, in the light-emitting step, the power source voltage VSS is set to −VEE, and the voltage of the source electrode of the drive transistor TD becomes a voltage VEE+VEL obtained by adding the power source voltage VSS and the ON voltage of the organic EL element EL. The voltage which is (1−α)ΔVdata+Vth−(VEE+VEL) is applied between the gate electrode and the source electrode of the drive transistor TD.
As a result, a current ipix=β/2((1−α)ΔVdata−(VEE+VEL))2, which does not include the term representing the threshold voltage Vth of the drive transistor TD, is supplied from the drive transistor TD to the organic EL element EL. Thus, the organic EL element EL emits light at the luminance corresponding to the magnitude of the current ipix.
As described above, the pixel circuit 90 reduces the effect of the threshold voltage Vth considerably by the Vth detection operation, and makes it possible for the organic EL element EL to emit light at a more accurate and stable luminance.